A second American infected with Ebola in Liberia arrived in the United States on Tuesday for treatment in Atlanta, and U.S. officials said they were sending more help to West Africa to fight the outbreak there.

Unlike Brantly, who walked into the hospital after being brought to the U.S. from Liberia on Saturday, Writebol was taken in on a stretcher, said Bruce Johnson, the president of SIM USA, the Charlotte, N.C.-based charity that sponsored Writebol's work in Liberia.

Although Writebol is "very, very weak," she is showing signs of "continued improvement," Johnson said at a news conference in Atlanta.

Writebol and Brantly last week were given doses of an experimental treatment developed by San Diego-based Mapp Biopharmaceutical Inc. Writebol, given the option of taking the drug while in Liberia, made the decision on her own, according to Johnson. He said it's not clear whether the treatment, dubbed ZMapp, is the reason Writebol is feeling better.

Her condition before taking the drug wasn't "dire," he said, but family and friends were "realistic."

Brantly's wife, Amber, said in a statement Tuesday that her husband was feeling better.

"I have been able to see Kent every day, and he continues to improve," Amber Brantly said. "I know that Kent is receiving the very best medical treatment available."

Johnson cautioned that the condition of Ebola patients often changes daily. Writebol was able to stand while boarding a plane back to the U.S., for example, but needed to be carried on a stretcher into the hospital Tuesday.

Writebol and Brantly will be kept in special isolation units at Emory, and treatment may last two to three weeks, doctors there said last week. It wasn't immediately clear whether they were still being treated with the Mapp drug.

Johnson said the cost of transporting and caring for Writebol during the trip from Liberia was almost $1 million and that the charity hoped the expense would be covered by its medical evacuation insurance policy.

Also Tuesday, the U.S. Agency for International Development said it is sending $5 million and a disaster response team to West Africa to help fight an outbreak that has killed at least 887 people in the region.

The funding will be used to "help trace people who may be infected, as well as provide health clinics and households with hygiene kits, soap, bleach, gloves, masks, and other supplies," USAID said in a statement.

The U.S. Centers for Disease Control and Prevention in Atlanta has said it will send an additional 50 medical workers to West Africa over the next 30 days.

In New York, meanwhile, doctors at Mount Sinai Hospital were awaiting test results from the CDC on a man who showed up Monday in their emergency room with symptoms of the disease after previously being in West Africa.

Although city health officials said it was "unlikely" the man has Ebola, they said a definitive ruling on blood samples sent to the CDC for testing would probably take 24 to 48 hours.

Ebola is spread between humans through direct contact with bodily fluids, including blood, urine and saliva. The disease, first reported in what is now the Democratic Republic of Congo in 1976, can cause bleeding from the eyes, ears and nose.

The outbreak that began in March is largely contained to the West African countries of Guinea, Liberia and Sierra Leone. One man, a 40-year-old American of Liberian descent, died of the disease in Nigeria after flying from Liberia's capital to the megacity of Lagos.

Nigerian health authorities acknowledged Tuesday that they did not immediately quarantine Patrick Sawyer, announcing that eight health workers who had primary contact with him were in isolation with symptoms of the disease.

Initially authorities said the risk of any exposure to others was minimal because Sawyer was whisked into isolation after arriving at the airport with symptoms of Ebola.

But Lagos state Health Commissioner Jide Idris said Tuesday that the nature of his disease "was not known" the first day, and only after further investigation did they suspect Ebola. Sawyer's sister had died in Liberia from the disease, which has no proven cure or treatment.

A doctor who provided care to Sawyer has tested positive for the disease, and seven other health workers showing symptoms were placed in isolation. They are among 14 people who had "serious direct contact" with Sawyer, most of them at the hospital, Idris said.

Authorities said they are monitoring the conditions of 56 other people who had contact with Sawyer.

Information for this article was contributed by Marie French and Michael Sasso of Bloomberg News and by Maram Mazen of The Associated Press.